One type of a DC-DC converter is disclosed in JP 2003-174768A, for example. As shown in FIG. 5, this DC-DC converter 10 includes: an inductor 111; a diode 112; a capacitor 119; a MOSFET 113a; a resistor 120; a switching control circuit 115; and an MPU 22.
One end of the inductor 111 is connected to the positive side of a battery power supply 2 through a positive input terminal 10a, and the other end is connected to the anode of the diode 112. The negative side of the battery power supply 2 is connected to a negative input terminal 10b. The cathode of the diode 112 is connected to one end of the capacitor 119 and a positive output terminal 1c. The other end of the capacitor 119 is connected to a negative output terminal 1d. 
The drain of the MOSFET 113a is connected to the junction of the inductor 111 and the diode 112, its gate is connected to the switching control circuit 115, and its source is connected to the negative input terminal 10b and the negative output terminal 1d through the resistor 120. Two input terminals of the switching control circuit 115 are connected to the MPU 22, and its output terminal is connected to the source of the MOSFET 113a. 
One input terminal of the MPU 22 is connected to the positive side of the battery power supply 2, another input terminal is connected to the junction of the diode 112 and the capacitor 119, and further input terminal is connected to the junction of the source of the MOSFET 113a and the resistor 120. Two output terminals of the MPU 22 are respectively connected to the switching control circuit 115.
The MPU 22 generates a basic switching signal for switching the MOSFET 113a based on the comparison between a basic step-up signal stored in an internal ROM 226 and a triangular signal. A duty factor of the basic switching signal changes based on the result of comparison among the voltage of the battery power supply 2, the voltage of the capacitor 119, a current flowing to the inductor 111 through the MOSFET 113a, and a standard change curve stored beforehand.
The switching control circuit 115 switches the MOSFET 113a based on the switching signal from the MPU 22. When the MOSFET 113a turns on, a current flows from the battery power supply 2 to the inductor 111, and magnetic energy is stored in the inductor 111. When the MOSFET 113a turns off, the magnetic energy stored in the inductor 111 is discharged through the diode 112, and the capacitor 119 is charged with this discharging current.
At this time, since a voltage is induced between the terminals of the inductor 111, the voltage of the capacitor 119 becomes higher than that of the voltage of the battery power supply 2. Thus, the DC-DC converter 10 steps up or boosts the voltage of the battery power 2 to a stable, specified high voltage and outputs it.
This DC-DC converter 10 generates switching noise according to the switching of the MOSFET 113a. The frequency of the switching noise corresponds to a switching frequency of the MOSFET 113a. This switching frequency is uniquely determined depending on the frequency of the triangular signal stored in the ROM 226 of the MPU 22. Therefore, the energy of the switching noise concentrates at the switching frequency. As a result, the energy of the switching noise in the switching frequency increases and may influence the operation of an electronic controller and the like disposed in the vicinity of the DC-DC converter 10.